Communication system employing pulse code modulation



J. R. PIERCE Oct. 12, 1948.

COMMUN I CATI ON SYS TEM EMPLOY ING PULS E CODE MODULAT I ON 4Sheets-Sheet 2 Filed July 9, 1945 lNVENTOR By J. R PIERCE ATTORNEY Oct.12, 1948 J. R. PIERCE 2,451,044

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed July 9, 19454 Sheets-Sheet 3 l 4W lll'r 5 WM ML:

5, F & P

I I I I Q o Z' F'I "a INVENTOR J R. PIERCE A TTORNE V J. R. PIERCE2,451,044

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION 4 Sheets-Sheet 4Oct. 12, 1948.

Filed July 9, 1945 lNl ENTOR By J R P/ERCE aw. J/L' LW.

ATTORNEY Patented Oct. 12, 1948 COMMUNICATION SYSTEM EMPLOYING PULSECODE MODULATION John R. Pierce, Millburn, N. .L, assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application July 9, 1945, Serial No. 603,934

14 Claims. 1

This invention relates to a communication systern for the transmissionof complex wave forms of the type encountered in speech, music, sound,mechanical vibrations and picture transmission by means of code groupsof a uniform number of signal pulses of a plurality of different typesor signaling conditions transmitted at high speed.

The object of the present invention is to provide a communication systemcapable of transmitting and reproducing a complex wave form over anelectrical transmission path in such a manner that the signal-to-noiseratio of the received signal is materially improved.

Another object of this invention is to provide improved and simplifiedapparatus capable of transmitting and receiving signal pulses over achannel having a low signal-to-noise ratio and deriving therefromsignals having a high signal-- to-noise ratio.

More specifically, it is the object of the present invention to providecircuits and apparatus for transmitting in succession a group of pulsesin sequence over a given channel representative of the amplitude of acomplex wave at successive instants of time, all as part of a system,sometimes identified as PCM or pulse code modulation.

Still another object of the present invention is to transform a seriesof pulses representing the amplitude of a complex Wave at a giveninstant of time into a single pulse having an amplitude which is afunction of the amplitude of the original complex wave at the giveninstant.

Another object of this invention is to recombine a succession of suchsingle pulses of varying amplitude in a manner to reconstruct a waveform of substantially the same shape as the wave form to be transmitted.

A feature of the invention is that of accomplishing the above objectswith a small number of pulses requiring a minimum of apparatus andequipment, taking advantage of the considerable distortion permissiblein speech without loss of intelligibility. The invention is similar insome respects to my copending applications, Case 28 and Case 29, SerialNo. 592,961, filed May 10, 1945 and Serial No. 603,989, filed July 9,1945, respectively.

Other features of the invention relate to synchronizing and coordinatingthe various circuits and equipment at the transmitting terminal witheach other and with the circuits and equipment of the receiving end soas to secure proper operation of the entire system.

Briefly, in accordance with the present invention, equipment is providedfor generating a control pulse or a group of pulses of predeterminedtime relation one with another. These control pulses are employed tocontrol a code element timing circuit, which circuit in turn generates aseries or cycle of very short pulses some of which are positive and somenegative and some a combination of the two.

Apparatus is also provided for rapidly sampling the Wave function andderiving an electrical quantity which is a function'of the amplitude ofthe complex wave to be transmitted, this sampling means being undercontrol of the control pulse generator. code element timing circuitgenerates the series or cycle of code element timing pulses referred toabove and these in combination with the sample amplitude derive theelectrical quantity of a magnitude related to the magnitude of thecomplex wave at the time of the control pulse.

The electrical quantity takes on the character of a voltage drop over animpedance which drop is increased step by step and is tested after eachaddition to see if it is below or in excess of the sample amplitudevoltage. If below, then the last addition is allowed to remain and if inexcess, the last addition is removed. This form of comparing the samplevoltage with the drop over the impedance is then carried on step by 7step in smaller and smaller steps to as far a point as may be desiredand in any case to an extent so t-hat the granularity of the signalfinally produced at the receiving point will be Within the atedapparatus are provided whereby the received pulses are employed toproduce a. pulse having a magnitude proportional to the magnitude of thecomplex wave sample at the transmission end of the system and thus thecomplex wave is reconstructed from a succession of such reproduced wavesamples.

A special feature of the transmitter comprises a means of arriving at .abinary representation of amplitude digit by digit, the largest digitbeing For each of the control pulses the 3 obtained and transmittedbefore the smaller digits are known. To achieve this, double triodeswith large cathode feedback resistors are used in switching on, andperhaps off, successively several constant reference currents in such amanner that the magnitudes of the currents are little effected by theswitchingoperation. Theprocess of switchingthe curreritofi automaticallytransmits the proper digit for each positional notation place. Aresetting pulse may be used also for transmitter blanking and sampling.The-voltage difierence between the sample and a voltage drop caused bythe double triodes acts through a cathode follower tube to provide abias:resulting in the selection of proper digits. turning one doubletriode off and the next on succeed one another and are of ippositepolarity so they may be obtained by producing two pulses of oppositesign by means of a network which produces a wave whose form is that of aderivative :of the wave .formof the single pulse from the :pulser.

.-A' feature=of the receiver is theme of a double triodewith a largecathode feedback resistor in combination with thedischarge of acondenser through a .resistor to change successive equal pulses, equallyspaced, into .pulsesea'ch having one half the amplitude of the precedin:pulse. Another .:feature ;is the use of .asingle pulse to (1-)"recharge to a constant initialvoltage acondenser which then gives 1 anexponentially decayi g voltage to a double itriode, (2) to discharge theaccumulated pulses .from .the double triode through an output filter.

A -feature of this system as a wholeqis that the measurement of thesample voltage is on a linear basis, 1. e.'the minimum variationin thesampled wave necessary to produce a significant variationin .the code reresenting the sample is the samemagnitucle forrlargeas forsrnallamplitudes of thesampled wave intead of being the same percentage ofvariationas in my application Serial No. 6 37,386, filed December27,1945, now :Patent ;No. 2,437,707, granted March 16, 195:8.

flhczinvention, both as to its organization .and method of operationtogether with other objects and features thereof, will be -'betterunderstood from .the following description taken with the accompanyingdrawings, in which:

Elias. 1.,and2 showin functional block form the various element and thelmanner in which they cooperate toform an exemplary communication systemembodyin the present invention;

Fig. 3 illustrates the timing and nature of the pulses characteristicofthe system at-the transmitting end;

.Figs. .4 to .6 give .in detail .the various circuits and operations ofan exemplary system embodying thepresentinvention, and

,Fig. 7 .is.a modification .of aportion of Fig. 4.

Referring more specifically to Fig. 1, let M be asignal functionrepresentative of any complex wave suchasa speech wave,'.a.small portionof which is indicated .by curve 300 o Fig. 3. A pulse generator .Gi setsup 'a plurality of .cycles of pulses, the cycles coming in rapidsuccession, perhaps about 8,000 per second. The first pulse of a cycleactivates .an electronic switch whereby a storage condenser .is chargedto *a potential equal to the amplitude of the wave function at theinstant of 'thepulsc. The remaining pulses in a cycle operate incoordination with the potential on'the storage icondenser *upona coder,as a result of which there The pulses :ior

,pf these. however, is dependent on a pulse generatorshown in Fig. 5 andsince the pulser op- .erationis independent of the circuit of 4 itwillbe advantageous to describe it first.

Q'I'he controllingelement in thi portion Of the .systemis arelaxationoscillator comprising a gas tube 5E0. "-This relaxation oscillator is ofa form vwelhknown in the art and includes a resistance 5 for charging acondenser 5|2. Assuming that :the-condenser EU is discharged then onclosure of the circuit it-is charged at a rate determined .in part bythe resistance 5| I. When the pote ntialiof the condenser and the plateof tube .5l0 ,risesto .a firing .value the condenser suddenlydischargesthrough .the tube and resistonSlA. The-duration ofthe discharge is.short and-gives rise to a sharppositive pulse acrossresistor 151 4. {Ihe duration-of this pulse .and the rate at which it is ,followed byidentical pulses can be completely controlled by the parameters of thecircuit; in particular, by the values of the elements 5H, .5l2 and .5l4taken with the po tentialfof the gridof .thetube .5l0astdetermined bythe potentiometer v 5| 5. While several forms of relaxation oscillatormay be used-at this point the one shown is .simple and satisfactory.

' ThQpositit-e pulse' from 5M -is now used .to cpntrol'the emission ofpulses tovarious parts of the circuit. This. is.,accomplished byconnecting across the element till a delay circuitils made up of atransmission line, .an artificial transmission line .oranetwork osections of inductance and capacitance .in. seriatim. .Ihepositive.pulse travels down this line or network, the ,time of arrival at eachsection .being spaced ,in accordance with the parameters of the varioussections and givi ngfrise to corresponding .pulses going out overchannels' to 4 for purposes to be described. While it is not .essentialthateach of themulsesbe'uniformly spaced intime from the preceding and thesucceeding i ulses, thecircuits and apparatus of .theexmplaryembodimentdescribed in .detail herein have .been arranged .to transmitand receive uniformly spaced pulses. Theldelay network is terminated bya load 540 ofmrfoper value to suppress any reflected wave.

{The parameters of .the relaxation oscillator may be adjusted vso thatpulses are delivered across resistor 514 at any frequency desired. Forthe purpose of my invention itis preferred tolhave .a sampling frequencyhigher than that of the highest .frequencycomponent in the complex wave.toibe transmitted. If, for example, this ,wavezis to .beaspeech waveand it is desired to transmit all components .up to 3,000 cycles then itis desirable that therelshould be at least twosamples per .cycl ,forthis highest frequency component. .Asuitable value, therefore, for therelaxation oscillator frequency would be 6,000 cycles although a highervalue .or a lower value maylbeusedif desired.

By means of the delay circuit or timestick 5I6"on e"has .availabl'esatthe ends of the respective sections positive .pulses similar to thatinitiated in 5| '4 and spacedintime oneafter another a i e jmii e -ihe'ham n m th section of the timestick. These pulses will control theoperation of three units, A, B, C, in a manner now to be described.

At time im when the pulse'is formed at 5M it is transmitted immediatelyto tubes 520, 530 and 540 in the three units. The plate circuit of 520in unit A includes the cathode resistor 522 and anode resistor 5'23supplied from battery 524. Consequently at the time tm there will beavailable a positive pulse across resistor 522 and a negative pulseacross 523, to be used in the circuit of Fig. 4 as describedhereinafter. A similar circuit arrangement is provided in unit B andunit with the exception that the cathode resistor corresponding to 522is omitted. Consequently, the negative pulse across 533 and 543 may belarger than that across 523 in a manner to be desired as pointed outlater.

In the unit A there is also shown a triode 52l the output circuit ofwhich includes resistor 525 as well as the resistor 523 with battery 524in common with the plate circuit of tube 520. All of the tubes 52|, 53|,54|, as well as tubes 520, 530, and 540 are biased to cut offor so thatonly a small or substantially no current flows in their output circuitsunless a positive pulse or potential is applied to their control grids.On the arrival of the pulse from l4 at the point 2 on the timestick thistube 52! is activated, yielding another negative pulse across 523 and apositive pulse across 525. A similar circuit comprising tube 531 in unitB is activated when the pulse from 5| 4 reaches the point 3 on thetimestick and there is then available a negative pulse across 533 and apositive pulse across 535. Similarly when the pulse over 5|4 reaches thepoint 4 it activates the circuit of tube 54! to yield a negative pulseacross 543 and a positive pulse across 545. These pulses due to theactivation of tubes 52l, 53! and SM are of substantially the samemagnitude, and in general are less than the initial pulses due toactivation of tubes 520 530 and r 540. The character of these pulses andtheir timing is shown in Fig. 3.

Referring now to Fig. 4, there is shown a source of a complex wave to betransmitted, such as a speech wave. This source may comprise amicrophone 4H] and suitable terminal equipment 4! l and transmissionline to supply the signal function M to the transformer 4I3. Associatedwith the secondary of the transformer is a storage condenser 5 whichtends to take on a potential difference equal to that developed in thetransformer. However, since it is desired to sample the wave for briefintervals of time only, the circuit is provided with two diodes 416 and4". A biasing battery 4|9 normally prevents flow of current throughdiode 4!! so that the condenser 4" does not charge. A positive pulsegoing directly from the pulse generator resistor 5|4 activates for ashort time the tube 420 converting this into a positive-negative pulsein the secondary of transformer 422. The transformer is so poled thatthe positive pulse will render diode M5 momentarily conducting in spiteof biasing battery M8 and will therefore discharge the condenser 4l5.Immediately thereafter the negative pulse will render diode 4 l1conducting whereupon the condenser 4l5 will be Charged to a potentialdepending on the instantaneous amplitude of the complex wave andyielding a sample to be used as hereinafter described. On completion ofthe pulse the condenser 5 will hold the charge till the arrival of thenext pulse on tube 320. In parallel with the transformer 422 is thetransformer 424, the purpose of which will be pointed out later.

It is now the function in this system to send out a code group of alimited number of pulses characterizing the voltage across condenser 5,that is the amplitude of the complex wave at the sampling moment. Theinvention will be described in terms of three pulses per group. To thisend the coder previously indicated in Fig. l is used and the processconsists, in part, of c0mparing the voltage over condenser 4l5 with thatacross a resistor R0, the current therein being adjusted step-by-stepuntil the drop over it is equal to the voltage over M5 or as nearthereto as the steps defined permit.

A feature of this invention is the use of three pairs of triodes V1, V2,V3, each having a cathode resistor R1, R2, R3 respectively, common toits two tubes. The grid of the right-hand tube of each pair is connectedto an intermediate grounded point of the B battery 45!]. Normally, nocurrent is flowing through the left-hand tube of each pair. However,making the control grid of any one of these left-hand tubes positivewith respect to ground will give rise to a constant current through R0while making the same grid negative will turn the current off. Thecharacteristic of a pair of triodes connected as shown is that thecurrent flowing through its common cathode resistor tends to remainsubstantially constant. If a positive potential of sufiicient magnitudeis applied to the grid of the left-hand tube, the right-hand tube willbe turned off, all the current then flowing through R0. The cathoderesistors R1, R2 and R3 may be of magnitudes related to each other inany desired manner but are preferably so arranged that the currentthrough Re which V2 controls is twice that which V3 controls, while thecurrent which V1 controls is twice that which V2 controls. This isaccomplished by suitable adjustments of the cathode resistors and theapproximate adjustment is that in which R2 is twice that of R1 and R3 istwice that of R2.

The tubes are turned on and off by pulses applied through the pairs ofdiodes V4, V5 and V6. The diodes of a pair are oppositely connected sothat the one will pass a positive pulse and the other a negative pulseto the grid of the corresponding left-hand tubes of the pairs V1, V2 orV3. Positive pulses applied through the transformers P1, P2 and P3 willalways turn on the respective left-hand triodes giving rise to acorresponding voltage drop across resistor R0. These positive pulseswill also charge the respective condensers 433, 443, 453 which will re--tain the charges till a negative pulse arrives. Small negative pulsesapplied through transformers D1, D2 and D3 will turn the respectivetriodes oif, but, as will be explained later, only if the'drop across R0exceeds the sample potential difference on the storage condenser 415.When a triode is turned oil? the current flowing through the controldiode from the corresponding condenser 433, 443 or 453 acts throughtransformers N1, N2 or Na to send a pulse to the transmitter 483 andthis results in the transmission of a signal code pulse.

The operation of the transmitter end of the system may be understood nowby combining the actions of the pulse generator with the coder. Thesequence of operations is as follows: At the beginning of the cycle,determined by the gen eration of the positive pulse at 554, a negativepulse is developed across resistors 523, 533, 543

asa; result of the activation of tubes 520, 530- and 540. These negativepulses act throughthe transformers D1, D2 and D3 and the correspondingdiodes to turn off any of the lef thand tubes of the pairs V1, V2 and V3that may have remained conducting during the previous coding cycle. Atthe same time a positive. pulse is developed over resistor 522 andarrives at transformer Pi, being delayed slightly by the delay network:$1. This pulse then turns on the tube 432.

At the sametime also, the positive pulse from 5M acting on the grid oftriode. 420 gives a positive pulse over cathode resistor-Ml which in thesecondary of the transformer 322 is converted into a positive-negativepulse. The positive pulse. acting through diode 4l6 discharges condenser 425 as already described and the negative pulse thereafter,operating on diode 4H, permits the condenser M5 to be charged to apotential,

corresponding to the amplitude of the complex wave at that instant. Thecharge on the condenser 455 then remains substantially constant untilthe pulse of the next coding cycle arrives.

The negative pulses over resistors 533 and 543,.

operating through transformers D2 and D3, will be relatively largebecause of the absence of cathode resistors for tubes sac and 545 andthey will be of sufiicient magnitude to overcome the biasing action of atriode V1 and so turn off tubes 4 32 and 452. The function of V7 will bedescribed below. As stated, the positive pulse from resistor 522,operating through P turns on tube 432 but, because of delay Si, notuntil the initial negative pulses are completed. 1"he initial pulse mayalso be used, if desired, to blank the transmitter through transformer424 so that the strong negative pulses in D2 and D3 at this time do notresult in the transmission of a. signal.

The character and the timing of the pulses during one cycle are shown inFig. 3. In this figure 305 represents a small portion of the complexwave and the dots indicate the successive sampling times. Line M is anexpansion on, a time basis of one cycle interval an to tm+i, the firstnegative-positive pulse corresponding to the sampling at far and thesecond corresponding to th sampling at tm+1. The remaining lines showthe timing of pulses within a cycle in the various circuits as willappear below.

With the tube 432 turned on, tubes 442 and 45?. turned off and a samplevoltage stored in condenser 455, it is desired to compare this lat- ,ervoltage with the drop across resistor Re. To this end a triode V7 isshown with its grid connected to the one terminal of condenser 415. Withno current in R0 and no charge on M5, the grid of V1 is biased back bymeans of battery B2 to approximately ground potential in which casecurrent of a definite value flows through the plate circuit of V7. Thepositive potential of the upper end of cathode resistor 41a is suitablyadjusted and applied through resistors 4'", 412 and 4 33 to the cathodeof the respective diodes 435, 445 and 455. This provides the necessarypositive bias on these cathodes so that the charge on the respectivecondensers 433, 443, e53, does not normally leak off. Blockingcondensers 414, 415 and 416 prevent the shorting of a portion of thebattery B3 in the plate circuit of V1 through resistance 418.

When condenser 4l5 has a sample charge and a current flows through R0,the potential of the grid of V1 will be raised if the drop over R0 isless than the voltage over M5. Consequently, the.

enlarged positive potential fromresistor 4'lo w ill prevent negativepulses coming over transformers D1, D2 and D3 from turning offthecorresponding triodes 432, 442 and 452. If, however, the drop over Roisgreater than thevoltage over M5, the potential of the grid of V7 will belowered and the positive potential from resistor 410 will be decreased.A negative pulse will then turn off the last activated double triodeand, by the discharge of the corresponding condenser through atransformer N, a. code signal pulse will, be transmitted.

As pointed out above, after the initiation of the first pulse in thecycle, triode 432 will be turned on. Shortly thereafter, a negativepulse is appliedito D1 corresponding to the time of arrival of the pulsein, the timestick 5H5v at the point 2, thispulse operating throughtriode 52l to give a negative pulse across resistor 523; If the dropacross R0 is greater than the sample, then triode 432 is turned off anda pulse is transmitted through transformer N1 indicating a. zero in thehighest order binary position. This pulse is indicated in line D1 ofFig. 3.

When the negative pulse over 523 is set up, a positive pulse is alsodeveloped across cathode resistor 525. This operates through a delaynetwork- S2 to transformer P2 which, operating through its diode, turnson triode 422. Somewhat later tube 53! will be activated, giving rise toa negative pulse over resistor 533 which operates through transformer B2on tube 422. If the drop across R0 exceeds the sample on the condenserM5, tube 422 is turned 01f and a pulse ,5 is transmitted by way oftransformer N2, shown the second pulse in line D2 of Fig. 3. If not, V2remains on and no code pulse is transmitted.

With the activation of tube 53! a positive pulse is also developedacross oath-ode resistor 535' and this operating through delay networkS3 turns on triode 452 very shortly after the testing, of. the eifectofVz. Still later the activation of tube 55! develops a negative pulseover resistor 543 which operates through transformer D3 to test whetheror not 452 should be turned off.

From the above description, it is seen that a combination or code ofthree off or on pulses will have been sent to the transmitter 480, thecode group being representative of or characterizing the amplitude ofthe sample as given by the potential charge of condenser 4|5. This groupof pulses, all rendered of the same amplitude by clipping if necessary,is accordingly trans mitted toa remote receiving station.

In the exemplary system shown in the drawing the pulses are transmittedfrom the transmitting station to the receiving station over a radiochannel; This channel, may operate in the short wave, ultra short Waveor micro-wave regions where the Waves have quasi-optical properties.

The signals may be equally well transmitted overopen wire lines, cableconductors, a coaxial cable, wave guide, etc. or any combination of suchpath including a radio path. The transmission path may also includesuitable terminal equipment amplifiers, gain and phase control One ofthese is theceives a pulse at the beginning of each cycle. Thistransformer is so poled as to block the transmitter 480 during thepresence of the large negative pulses which turn off tubes 442 and 452on the initial pulse of each cycle.

The allowable range of operation of the circuit lies between a voltageon condenser 4l-5 which, when added algebraically to the voltage of B2will brin the grid of V7 and hence the drop across 410 to certainspecified values described above, and a voltage on condenser M5 which,when added algebraically to the voltage of B2 and the drop across R whenthe grids of tubes 432, 442 and 452 are positive, brings the grid of V7and the drop across 410 to the aforementioned values. It will beapparent that by a simple phase reversal in transformer 413 either ofthese voltages across M referred to above can represent the minimum ofthe AC signal. Furthermore, leav ing the circuit comprising 4"], 4| l,M3, M5, M6,

4H,- MS and 422 as they are, one may interchange the connections of theterminals of condenser 415 to B2 and the grid of V7. In this case thevoltage across the condenser and that across R0 are in the samedirection and the drop across R0 must be so adjusted by tubes V1, V2 andV3 that the sum of these two voltages will be constant within theoperating limits. This sum combined with a reduced B2 voltage will thenbe the required voltage of the grid of V7. The signal then transmittedrepresents the magnitude of the drop over R0 as before but is now thecomplement of the voltage across the condenser, i. e., it is equal to aconstant minus the condenser voltage. This complementary signal willcarry the same information but with a 180-degree phase reversal.

Receiver A satisfactory circuit for receiving and decoding the groups ofpulses from the transmitter station is shown in Fig. 6. In this figurethe message indicated as arriving on a radio carrier is detected down topulse frequency in any suitable terminal equipment BID and is then shownas being amplified by tube BIZ. The receiver circuit itself consistsessentially of a decoding portion and a pulse generating portion, thelatter of which will be described first. A positive pulse arriving atthe input of N2 is inverted and is transferred as a negative pulse tothe grid of a tube 62l which then inverts and amplifies it, ifnecessary, for application on the grid of a relaxation oscillatorcomprising the gas tube 623. This relaxation oscillator may be-of thesame general form shown in Fig. 5 and by means of the resistor 524 andcondenser 625 is adjusted to substantially the same frequency as theoscillator of Fig. 5; that is, to give one oscillation for each cycle orgroup of pulses characterizing a sample. Furthermore its adjustment issuch that it is trigered off bya positive pulse of sufiicient magnitudeon the grid of 623. The positive pulse generated over cathode resistor62'! is transferred through condensers B3! and 632 to the decoderportion of the receiver circuit.

This decoder comprises the double triode V9 wth the common cathoderesistor 634. The plate circuit of the left-hand section of V9 includesthe condenser 642. A resistor 64! may be included, but not necessarily,in parallel with condenser 642. The bias of that section is such that itis at or below cut-off. If, however, its grid receives a positivepotential its current will rise at the expense of the current intheright-hand section. If an on pulse arrives from the transmittingstation, the relaxation oscillator is triggered, a positive pulse ofshort duration is impressed on the grid of tube and a charge is storedon condenser 652 which condenser then discharges exponentially throughcathode resistor 653. The potential of the condenser 652 is impressed onthe grids of both sections of V9 and it will be 2 observed that this isalways a positive potential which, however, will decrease exponentially.The

circuit is so arranged that the potential across the condenser 652decreases by half its value in the time interval between successivedigit pulses. If at the beginning of this cycle there is received an onsignal then a positive pulse appears through transformer 635 on the gridof the lefthand section of V9 and the potential of this grid s thereforeequal to the potential of the pulse minus the drop through cathoderesistor 634 plus the potential across condenser 652, plus anyadditional bias 636. A pulse of short duration and proportional to thisgrid voltage will flow through the left-hand section of tube V9 andcharge the condenser 642 correspondingly. Another pulse of a code grouparriving at a later time will give rise to an additional pulse throughV9 and an addition to the charge on condenser 642. But, although theamplitude-of the arriving pulses will be the same, the potential of thecondenser 652 will have decayed so that the charge received for a pulsecorresponding to tube V2 at the transmitter will be less than that forone corresponding to tube V1 and for a pulse corresponding to tube V3will be still less, since the potential on condenser 652 will havedecreased to a still lower value. At the conclusion of the cycle,condenser 642 will have a charge determined by the particular code groupwhich has arrived. Then with the beginning of the next cycle a pulsewill be transmitted from tube 623 through condenser 632 and transformer63! to the grid of tube 66], normally below cut-off. Arrival of thispositive pulse, however, activates the tube and a pulse from condenser642 will flow through the plate circuit of tube 66!, the quantity of thecharge passin being equal to the charg on condenser 642 and thereforesubstantially proportional to the original sample.

The code groups will follow each other at a rate equal to the samplingrate at the transmitter and these may then be passed through thelow-pass filter 610 into suitable terminal equipment 680 and appropriatereceiver 68! (here shown as a head-set) in which there will thus bereproduced th original complex Wave function.

It will be evident that the tube 623 may be triggered off by any pulsein a code group inasmuch as they all arrive with the same amplitude, andthe particular place in a code cycle where the tube will be triggeredoff is one of chance depending on when the receiver circuits are closed..It is important, however, that its triggering shall be brought intocoincidence with the first pulse in a cycle. To this end an additionalfeature is included with the relaxation oscillator. This may comprise atube BSI, here shown as a pentode. Its output circuit is connectedacross the condenser 625. Its input circuit comprises a battery 692 anda key 693 in series with an in-. ductance 694, paralleled by apotentiometer resistor 695. There may also be included in series acurrent limiting resistor 696. On closure of the key 693, a. largevoltage appears across the inductance 694 which voltage, however,rapidly falls as current through 694 becomes established. The connectionis such that the positiveend of -storage condenser.

11 the inductance, operating through potentiometa- 695, impresses apositive voltage on the grid of pentode 69! for a moment only. Duringthis interval charge will be withdrawn from 625 delayingthetime whenthetube 623 is again ready tobe triggered oil. The magnitude of thepulse ap'plied'to the grid of SM is adjusted so that the delay intriggering time is approximately onethird of the cycle period, i, e.about one digital period. By at most two or three taps of the key 693the triggering of tube 623 may be brought into coincidence with thearrival of the first pulse in a cycle. The opening of key 693 will giverise to a large reverse voltage over inductance 694 but this has noefiect inasmuch as it drives the grid of pentode 691 to a negativevalue.

Normally the relaxation oscillator at the receiver will be held insynchronism with the transmitter by the first digit pulse in each cycle.If the first digital signal of a cycle is an off-pulse, the relaxationoscillator will then trigger off itself and sufliciently closesynchronism will be maintained, if there is not an excessive number insu'ccession of off-signals for the first digit.

The'arrangement of the diodes M6 and 4H as shown in Fig. 4 is such thatthe condenser 4| 5 will 'alwaysreceive a negative charge on the topplate,'so that the potential from this condenser plate to the grid of V7is positive. This, however, is not necessary. By reversal of thedirection of the diodes and their biasing batteries and a reversal'oftransformer 422 the condenser will be charged in the reverse directionand appropriate behavior of the circuit is still obtained by-a com- 'anoperative system requiring as few and simple parts as possible.

Fig. shows an alternative input to that shown in-Fig. 4. In thisalternative form a pentode Vs replaces V7 and the storage condenser isconnected across the input circuit, which input circuit includes-the=cath'ode resistor 'liil. The plate circult ofthis pent'ode includesthe resistor R and a fairly large resistor 103. The screen grid isconnected to'positive'of battery; that is, to the positive end of Ru.Normally, current flows through'the pentode of su'fiicientmagnitude so"that considering the'drop through R0 and 103 the potential of the plateis slightly positive and is directly connected to-the resistors 4', "412and- 41-3, with the elimination of V7 and its circuit.

With no signal on the storage condenser and with the double triodesturned ofi the potential of the cathodes of the left-hand sections ofthe diodes is-sufficiently positive so that the negative pulses comingup through the D transformers will not render the said diodesconducting. A signal potentialon the storage'condenser, lowering thepotential of the control grid, will raise the potential of the 'plateand the corresponding diode cathodes.

What is claimed is:

1. In a communication system apparatus responsive to permutation codegroups ofpulses of signaling conditions, a relaxation oscillator,apparatus responsive to a particular-pulse of each of said code groupsof pulses for accurately controlling the frequency of said relaxationoscillator, and other apparatus for shifting the controlof saidrelaxation oscillator from one of said Pulses of each code group toanother of the pulses of each group.

2. In a communication system apparatus responsive to permutation codegroups of pulses of signaling conditions, a relaxation oscillator,apparatus responsive to a particular pulse of each or said code groupsof pulses for accurately controlling the frequency of said relaxationoscillator, and other apparatus for shifting the conirol of saidrelaxation oscillator from one of said pulses of each code group toanother of the pulses of each group, decoding apparatus for decodingsaid permutation code groups comprising apparatus for varying theefiectiveness of each of the pulses of a code group and other apparatusfor adding said changed pulses of each code group together, and meanscontrolled by said relaxation oscillator for generating a-single pulsehaving a magnitude proportional to the sum of said added pulses.

3. In a communication system in which the amplitude of a complex wave issampled at recurrent intervals and the sample amplitude epresented by apermutation code group of pulses each of either of two difierentsignaling conditions for transmission, means for storing a volt-ageproportional to the sample amplitude of the wave, means for building upa reference current of components of successively smaller amplitudes,the number of components corresponding to the number of pulses of saidgroup, means for comparing the stored voltage with the voltage producedby the flow of the reference current through an impedance upon theaddition of each component current, and means responsive to said meansfor comparing for establishing the signaling condition of each of saidpulses.

l. A combination according to claim 3 in which the means for comparingthe stored voltage with I the retention of that component referencecurrent-lastadded only if the sample voltage is in excess ofsaid-voltage produced by the-flow of the referencecurrent throughan-impedance.

5. In a communication-system in which the amplitude of the intelligencewave to be transmitted is sampled at recurrent intervals and the sampleamplitude represented by a-series of permutation code pulses each ofoneof a plurality of different signaling conditions, means for storing avoltage proportional tothesample amplitude of the intelligence wave,-aresistor, a; series of constant current devices -for controlling 'theflow ot-current through said-resistor, means-for actuating all of saiddevicesto a first-condition, means for subsequently operatin said idevices -in succession to a'secondcondition; means responsive to therelative values of the -stored-voltage and of the voltage drop acrosssaid'resistor for controlling the restoration of each of said devices-tosaid-first condition prior to'the actua- 13 tion of the next device ofsaid series to said second condition, and means also responsive to therelative values of said voltages for controlling the signaling conditionof the respective pulse of said group.

6. In a, communication system in which the amplitude of an intelligencewave is sampled at recurring intervals and each sample amplituderepresented by a group of permutation code pulses each of one of aplurality of different signaling conditions, means for storing a voltageproportional to the sample amplitude of intelligence waves, a resistor,a series of constant current devices for controlling the fiow of currentthrough said resistor, the number of said devices correponding to thenumber of pulses of said group, a flesistor, means for actuating all ofsaid devices to a first condition, means for subsequently operating saiddevices in succession to a second condition, means responsive to therelative values of the stored voltage and of the voltage drop acrosssaid resistor for causing the restoration to said first condition of thelast of said devices operated to said second condition when the voltagedrop across said resistor exceeds the stored voltage and prior totheactuation of the next device of said series to said second condition andfor maintaining said last operated device in said second condition whensaid stored voltage exceeds said voltage drop across said resistor, andmeans also responsive to the relative values of said voltages forcontrolling the signaling condition of the respective pulse of saidgroup 7. In a communication system in which the amplitude of anintelligence wave is sampled at recurring intervals and the sampleamplitude represented by a series of pulses of one of two differentsignaling conditions, means for storing a voltage proportional to theinstantaneous amplitude of the intelligence waves, a resistor, a seriesof constant current generators connected to supply current to saidresistor when actuated, means for actuating said generators in turn toeach produce a difierence voltage drop across said resistor, means forcomparing the voltage drop across said resistor with the stored voltagefor producing a control voltage when the stored voltage exceeds the dropacross said resistor, means operating after the actuation of eachgenerator and before the actuation of the next generator of the seriesfor rendering inactive the generator last actuated except in thepresence of said control voltage, and means responsive to said controlvoltage for transmitting a pulse of one of said signaling conditions.

8. In a pulse code modulation system, means for reourrently sampling theWaves to be transmitted to produce a voltage proportional to theamplitude thereof at each sampling instant, a resistor, a series ofsources of constant current connected to supply current to said resistorwhen operative, the current supplied by said sources to said resistorbeing so proportioned that the voltage drop across said resistorincreases as the sum of the ascending powers of two as successivesources of said series are rendered operative, and timing means for:first, rendering all of said sources inoperative to supply current tosaid resistor; second, rendering a first source of said seriesoperative; third, rendering said first source inoperative except whenthe voltage proportional to the sample is greater than the voltage dropacross the resistor and transmitting a pulse of one characteristic uponthe rendering of the source inoperative; and repeating said second andthird steps for each source of said series.

9. In a communication system in which the amplitude of the intelligencewave is sampled at recurring intervals and the sample amplituderepresented by a permutation code group of pulses of either of twodifferent signaling conditions, means for storing a voltage proportionalto the sample amplitude of the intelligence wave, a resistor, aplurality of electronic devices each connected to supply a respectiveconstant current through said resistor when operative, means fornormally maintaining each of said devices inoperative, a controlcapacitor associated with each of said devices and adapted when chargedto maintain the corresponding device operative, a discharge circuit foreach of said control capacitors, means for supplying to said dischargecircuits a biasing voltage for preventing the discharge of said controlcapacitors except when the voltage drop across said resistor exceedssaid stored voltage and a control pulse is applied thereto, and timingmeans for charging said control capacitors in succession.

10. In a communication system in which the amplitude of the intelligencewave is sampled at recurring intervals and the sample represented by apermutation code group of pulses of each of either of two signalingconditions, means for storing a voltage proportional to the sampleamplitude, a resistor, a series of constant current devices connected tosupply to said resistor component currents producing across saidresistor valuation voltages that decrease in proportion to thedescending powers of two as said devices are individually switched froma normal to an operated condition in succession in said series, meansfor switching the devices of said series from a normal to an operatedcondition in succession, and means operated after the switching of eachdevice of said series from a normal to an operated condition and priorto the switching of the next device and responsive to the relative valueof said sample voltage and said valuation voltage for restoring thedevice last switched to the normal condition or for maintaining it inthe operative condition dependent on the relative value of saidvoltages.

11, A combination according to claim 10 in which the last-mentionedmeans operate to maintain the device in the operative condition whensaid sample voltage exceeds the valuation voltage.

12. In a communication system in which the amplitude of the intelligencewave is sampled at recurring intervals and the sample amplituderepresented by a code group of pulses each of either of two differentsignaling conditions, means for storing a voltage proportional to saidsample amplitude, a resistor, a series of constant current devicesnormally non-conductive and each connected to supply a component currentto said resistor when conductive, a capacitor for each of said devicesadapted when charged to maintain the respective device conductive, acircuit for charging each of said capacitors in response to a controlpulse, a circuit for discharging each of said capacitors in response toanother control pulse, means responsive to the relative voltages on saidmeans for storing and across said resistor for rendering said circuitfor discharging nonresponsive to said other control pulses when thevoltage on said means for storing exceeds the voltage drop across saidresistor, means responsive to the operation of said circuit fordischarging said capacitors for producing a signal pulse of '15 onecondition, and timing means for producing a series of pulses comprising:first, a clearing pulse tooperate all of said circuits'for dischargingirrespective of the relative voltages of said means for storing andacross said resistor; second, a control pulse for charging the capacitorof the first constant current device of said series to render saiddevice conductive; third, a control pulsefor discharging saidcapacitors; and subsequently corresponding control pulses for each ofthe other 10 constant current devices of said series.

13. A combination according to clair .12 in which the component currentssupplied by said series of constant current devices decrease inproportion to the descending powers of two.

'14. A combination according to claim :12 in which each of said constantcurrentdevices comprises a pair of electron tubes eachhavinga cathode,an-anode and a control electrode, :a

resistor common to the cathode circuits of:.sa-id tubes, a controlelectrode circuit for one:of.said

tubes'for maintaining that tube normally conducting, connections fromthe anode of the'other of said tubes for completing the circuit to saidresistor in the so-called conductive condition of 16 the *device, andconnections vfrom said capacitor forxeac'h .of said devices-to supply avoltage tto thegrid of said other device to render it conducting.

JOHN R. PIERCE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES :PATENTS OTHER REFERENCES Review of scientificInstruments, vol. 9, :March 1938, pages 83-89.

